Microwave coaxial termination



March 30, 1965 v. J. MCHENRY 3,176,248

MICROWAVE COXIAL TERMINATION med sept. 12, 1962 INVENTOR.

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United States Patent O kThis invention relates to apparatus for absorbing the incident power of electromagnetic waves within the microwave spectrum and also relates to the method of malcing such apparatus.

In many instances it is necessary to have a device terminating an electrical circuit capable `ot absorbing incident power over the entire microwave spectrum, i. e., 1000 megacycles per second to 12,080 megacycles per second, with substantially no reflection of the incident wave'. One application for such a terminating device is where it is desired to determine or measure various characteristics of microwave equipment, ie., as tor a coupling, in which it is necessary for test purposes that there be substantially no reilected energy from a particular propagation path. ln some microwave apparatus, ports are provided across various stages of the circuits to allow measurements to be made at `t ese points. Another example of use for the microwave termination would be to terminate these ports. it is an object of this invention to provide a novel microwave termination for substantially absorbing the incident power of electromagnetic waves within the microwave spectrum.

One type of microwave termination is of a coaxial construction having a center conductor and an outer shield with a lossy or microwave absorbing materiall disposed between the center conductor and the outer shield. the amount of reflected energy from the microwave termination must be maintained at a minimum, good impedance-matching must be obtained between the termh tion and the line with which itis associated. To facilitate impedance-matching, it has been conventional practice to form the lossy material in the shape of a long taper, tapering into substantially a knife-edge at the input end, with the length lof the taper being approximately equal to two wave lengths of the lowest frequency within the microwave spectrum to be absorbed. While such a construction is eitective, it does necessitate the use of a termination which is rather long. lt is an object ci this invention to provide a microwave coaxial termination of a rela- `tively small size for substantially, completely absorbing,

with minimum reection, incident power ot electromagnetic waves within the microwave spectrum.

In order to more completely absorb microwa 'es of a particular frequency, it has been the practice to construct terminations of the long taper type, previously mentioned, i-n which the tapered lossy material or load is movable axially relative to the shield and center conductor. By properly adjusting the location of the load along the line, the point of maximum absorption can be found for that given frequency. It is an object of this invention to provide a novel termination for absorbing the incident power of electromagnetic waves within the microwave spectrum in which means are provided for accurately adjusting the termination for maximum absorption of a desired frequency.

Itis a further object of this' invention to provide a novel method for ymaking the microwave termination of the above described type.

Other objects, features, and advantages oi the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction f with the accompanying drawings inwhich:

latenteel Mar. 30, i955 FGURE l is a side elevational, sectional view, with some parts shown bro en away, oi a microwave termination exemplifying the features of this invention;

FIGURE 2 is a side elevational, sectional view, with some parts shown broken away, of a termination of modilied construction, exempliiying the features of this invention; and

FlG-Ull 3 is a fragmentary, side elevational, sectional view depicting a modification of the termination shown 'n FlGURE l. v

Looking now to FlGURE l, a coaxial termination, exemplitying the features of this invention, is generally indicated by the numeral lll and has a coaxial conductor I Z having input end portion ld and a coiled, termination end portion The coaxial conductor l2 can be of a conventional construction having a center conductor lll surrounded by an outer electrical conductor or shield l with the conductors fil and i3 separated from each other by an electrical insulator l5. The termination portion "lo is coiled or wound in the form of a helix tapering radially inwardly while extending axially from its input end (connected to input end portion M) to its opposite end. For the salte of convenience, the coiled portion l5 can be thought or" as dell. .g radially inner and outer surfaces. Thus the outside diameter along the radially outer surface of the termination portion lo varies from a maximum at its input end to a min mum at its opposite end. Normally, of course, the radially irner surface of the coiled termination portion lr6 would be of a similarly tapered configuration. However, for a purpose to be presently seen, the radially inner surface of the termination portion ld is cut substantially to a uniform diameter D. Thus, with the termination portion ld, the outer shield i3 is provided with an opening which faces radially inwardly, with the opening being a minimum proxima e the input end of the termination portion lo and progressively increasing to a maximum at the opposite end. ln the embodiment of FlGURlE l, the terminus of the coaxial coil l2 at the opposite end of the termination portion lo has been reduced to one-half its original cross section; note that, in reducing the cross section of the line l2, both the shield i3 and center conductor ll are nartially removed to provide more effective power dissipation. For practical reasons, the cross section is not reduced greater than one-half, since, with less than one-halt of the coaxial line l2 remaining, the mechanical connection maintaining the shield l, insulator l5, and center conductor ll. together would be broken.

ln a preferred form, the total straightened length of the coi ed termination portion lo is approximately equal to twice the wave length oi the lowest frequency of the microwave spectrum to be absorbed, i.e., for a termination to absorb the entire microwave spectrum the straightencd length would be approximately equal to twice the wave length of the i900 megacycle per second wave.

The coaxial line l2 is located in a tubular housing ld constructed of metal or plastic and having a generally circular cross section; in tue embodiment of FGURE l the housing ld is made of a material which is a good conductor, i.e., high conductivity, of electricity. The housing i3 is of a substantially uniform diameter in the region ot the termination portion ld tapering at one end to a reduced diameter neck portion l@ which is of a diameter to matably accommodate the terminus of the coaxial line l2 at the input portion ld. An epoxy resin, indicated by the numeral Eil, can be applied between the housing and the outside diameter of the terminating portion l5 to thereby secure the coaxial linel?I witln the housing The opposite end of the housing 118 can be sealed by a cap 2.2. The terminus of line l2 at the V mum.r

intermediate portiony 16 is s ly connected to the cap 22.

Secured by aretainer ring Z3 to the necked portion 19 of the housingi18 is a threadedjfeniale connector 24. The terminus of the-inputportion 14'of the coaxial line y12 has the centerconductor 11 extending axially outwardly( beyond the' shieldl and .insulationA 15 and is oldered or otherwise electrical- Y 28 vand is accessible through an elongatedwindow or slot 36 located in the extension Yportion 32. '.Thus the knob Y 34 can be manipulated by hand to cause the'v lossy core matably engageable withA a'suitablefemaleV connectornot ,Y

' shown)` of artype wellknown in thea art.-

A core of lossy material 26 is locatedin the inside dif' ameter ofthe-terminating portion 16 and canjbe made of pressed carbon, a. ferromagnetic loss material, or some other type oflossyv material well known'in theart. i i

FIGUREJZ depictsamoditication ofthe embodiment of FIGUREy 1 in which components having similar functions are given the samenumbers'with the addition Vof a letter subscript.. uInFIGURE 2 Vthe microwave coaxial ter#V mination 10a has a coaxial linelZa havingan'input end portion 14a and a termination portion 16a whichis'heli-` s cally Wound to form a coil tapering radially inwardly^ which extends axially in aV direction toward the inputend portion: 14a.'Y Thus the insidediameterof the Vtermination portion 16a varies froma minimumat the input end to amaximurn at the opposite end. f The outside diameter is Acut to a diameter DaA such,Y that the shield 13a' ispro;

'material 261]: to be moved axially relative-to the termination portion of the coaxialline 12b to thereby allow the termination lltlb'to be set to providefor maximum absorptionata particular frequency. s For the sake of convenience,v Vcalibration-marks rcan be provided along the side of the slot 36'and a markerindicated on the knob 34 wherebyjthe termination'ltlb can ybe calibrated for maximum absorption overa given frequence range. With the slidingtermination 10b, greater changes can be effectedrwith'less axial movement of theV core 26h than with sliding terminations of conventional,construction. Thus a savings in size is` realized. A similargadaption to va sliding' termination could 10a of FIGURE 2.

I y Y To make'the'termination 1l)k of FIGURE 1,- in a pre- 'ferred'embodimenn a coaxial line having a lengthV greater Vthan Vtwoy Waye lengths off-thelowest'frequency in the Y microwave'spectrum'to-l be absorbedis'obtained. Next, a portion of the'coil, substantially'equal'to the two-wave length dimensiornjis Wound into a helically, uniformly tapered coil to form theitermination portion; 16, with the large diameter endfbeing proximate thegremaining input portion le'ofgthe coaxial line. In general, the diameter f torwhich the coaxial 'line`v is wound is not critical, hence allowingrconsiderable variation lin the final physical dimensionrof the termination; e -f y/Next, the internal diameter ofv the'termination portion ofthe-'coil is cut-toa uniform'diameter'D, preferably l whereby the shield VY13 Yat the rirstcoil is just slightly vided with an opening` which faces radial1y^outwardly,

with the opening proximatethelinput end being a minimum and progressivelyr increasing toga maximum at the opposite'end. At the opposite end, the shield 13M` and center conductor-11m have been approximately cut in." half.

The terminating portion Marsan beprovided-with` abroken While thev crosssection-of the` last coil is'substantiallyreduced 'to one-half ofitsorigin'al cross section. In orderV to providethis relationship, the-coil is.` tapered such thatjthe inside diameter vofthe first coil -at the-large eend is just 'slightly less than the uniform diameter D and i the mean diameterI of thelast coil at the small end is center core of epoxy materialfZtia and is providedrwith a Y portion. Y vThe slug and"coaxial line, as'assembled, can

cylinder. of lossy material 26a which extends peripherally about the outside diameterV of. the terminatiorr'portion 16a., The coaxialline 12aY can be mounted in a'housing 18a which is similarrto housing'18. Again, inthe construction shown in FIGURE 2, the-microwave energyV traveling along the coaxial line! 12a is gradually, increas- Vtionsin the ydegree of V"absorption forditerent'frequencies;

is the position ,of the Ycore. of absorption materialtrelativel lthen be: assemb ingly exposed to the lossy/material 26a, thereby providing Y substantially equal-:to thediarneter D.-. f Y' Y l v Next,V a generally v.cylindrical'fslug- 26'of lossymaterial is-fpress-tted into the inside fdiameter of the termination led intoal case or'housng 18 having appropriate iittings.. Y 'l L f 'lhevmanutacturey of the termination 10a "of FIGURE 2 can be, accomplished in aV similarmanner except that, in the. embodiment. OFIGURE 2, a portion of a coaxial- VIineis `wound. into a tapered coil 16a having a gradually Y increasing diameterffrom its input endportiont14a.` Next;

the t outside diameter is machined to` a uniform diameter DE, whereby theamount of materialremovcdfrornfthe Y outside'diameteris a minimumy proximate .the input end Tand gradually increases, `Withthe. last-coil being substanto the shield Aand to `thepcenter conductor.- Ingorderlto allow theitermination torbesetfor maximum absorption. f

for a particular frequency, arrsliding termination canfbe,

utilized whereby therlolssyor absorbing materiall is moved s relative to the coaxial line. In' FIGURE 31am adaption. of the microwave termination construction of FIGURE l to aslidingtermination is shown in which similar components having similary functions are, 'given Ysimilanlnum-v t bers with a letter subscript added.'Y .In EIGUREBa slid V ing termination, only partially, shown,Y is generally indicated by the numeral 10b landhasa coaxial line 12b, of

a construction similar to thatofcoaxial line',v 12 ,'.ofiFIG/-e tially'cut in half.` v-Next, a hollow cylinder off lossy V material ist: press-iitted overfthe,outside-diameter Dl of' the termination portion 16a..` The` coaxial line-112e assembly c anthen be, assembled into anappropriate housing 18a. l

While in none Yof` theabove vconstructionsis the meanl diameter ofthe coiledterrninationportion '16, 16a critical,

fardiameter shouldbe-selectedjwhich facilitates-,a gradualtaper with-a gradual change in the amount ofexposure 'n ofthe termination portion 16, ,-16a, to the absorbingmember.2r6,i26a. l l

comparison: to a termina-y tion either iixedforsliding of ytheftapered4 absorbinglma-1 `It, can be appreciated that, in

ter'ial type, terminations'constructed in facordance with URE 1whichfis secured Ywithin an outer` case'or hous-i ing 18b. A core 2611 of lossy material is slidably dis- `posed withiny the inside diameter of thetermination 'portion 1Gb and 4has -a rod 2S secured at one .end-thereto.`

The opposite end lof rod.28sis slidably supportedfin 'auf f bushing 30 .supported in a coaxiall extension' portion 32' of the housing 1Sb. A knob 34 is secured to the rod change FIGURESgl, 2 and-3A are-*smallen-more compact,'and lend themselves toy a directmethodof construction. ls

lWhile it will be apparent that the'prefe'rred embodi f ments. of the invention disclosed arerwell calculated to yfuliillthe objectsabove stated,vit.vvill` be appreciated that*- the invention is snsceptibley to modiication; variation and meaning of the subjoined claims.

be provided for the termination without departing from .the `proper scope or fair- What is claimed is:

1. Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line, said outer shield of said coaxial line of said termination portion being partially removed to define an opening progressively increasing from a minimum proximate an input end of said termination portion to a maximum at an opposite end, and a member of microwave absorbing material disposed proximate to and in .confrontation with said opening.

2. Apparatus for absorbing incident power of electromagnetic Waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line, said coaxial line of said termination portion decreasing in cross sectional area gradually from one end of said termination portion to an opposite end with said outer shield of said coaxial line of said termination portion having an opening progressively increasing from a minimum proximate said one end of said termination portion to a maximum at said opposite end, an input portion of said coaxial line at said one end of said termination portion, and a member of microwave absorbing material disposed to and in confrontation with said opening,

3..Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line, said coaxial line of said termination portion decreasing in cross sectional area gradually from oney end of said termination portion to an opposite end with said outer shield of said coaxial line of said termination portion having an opening progressively increasing from a minimum proximate said one end of said termination portion to a maximum at said opposite end, an input portion of said coaxial line at said one end of said termination portion, a member of microwave absorbing material disposed to and in confrontation with said opening, and means operatively associated with said member and said termination portion of said coaxial line o for moving said member relative to said termination portion.

4, Apparatus for absorbing incident power of electromagnetic wavcs within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line defining a coil having inner and outer surfaces, one of said surfaces of said termination portion of said coaxial line having the material of said coaxial line partially removed with the amount of material removed from said one of said surfaces on adjacent coils gradually increasing from a minimum at one end to a maximum at the opposite end, an input portion of said coaxial line at said one end of said termination portion, and a member of microwave absorbing material disposed in peripheral proximity to said one of said surfaces of said termination portion.

5. Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line defining an axially extending, radially tapering coil having similarly tapering radially inner and outer surfaces, one of said surfaces of said termination portion of said coaxial line having the material of said coaxial line partially removed to a substantially uniform diameter with the amount of material removed from said one of said surfaces on adjacent coils gradually increasing from a minimum at one end to a maximum at the opposite end, an input portion of said coaxial line connected to said one end of said termination portion, and a member of microwave absorbf ing material disposed in peripheral proximity to said one of said surfaces of said termination portion.

6. Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line coiled about an axis with axially adjacent coils defining radially inner and outer surfaces, said coaxial line of said termination portion decreasing in cross sectional area gradually from one end of said termination portion to an opposite end with said outer shield of said coaxial line of said termination portion having an opening facing radially from one of said surfaces with said opening progressively increasing from a minimum proximate said one end of said termination portion to a maximum at said opposite end, an input portion of said coaxial line at said one end of said termination portion, and a member of microwave absorbing material disposed in peripheral proximity to said one of said surfaces of said termination portion.

7. Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a generally tubular housing, a coaxial line disposed in said housing and having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line coiled about an axis with axially adjacent coils defining radially inner and outer surfaces, said coaxial line of said termination portion decreasing in cross sectional area gradually from one end of said termination portion to an opposite end with said outer shield of said coaxial line of said termination portion having an opening facing radially from one of said surfaces with said opening progressively increasing from a minimum proximate said one end of said termination portion to a maximum at said opposite end, an input portion of said coaxial line at said one end of said termination portion having a terminus coextensive with one end of said housing and with said center conductor extending axially beyond said shield, connector means secured to said one end of said housing and operative with said terminus of said input portion to define an electrical connector, and a member of microwave absorbing material disposed in peripheral proximity to said one of said surfaces of said termination portion.

8. Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line coiled about an axis with axially adjacent coils defining radially inner and outer surfaces, one of said surfaces of said termination portion tapered radially between one end and an opposite end, the other of said surfaces of said termination portion extending axially substantially radially uniformly with said outer shield of said coaxial line being partially removed to define an opening facing radially from said other of said surfaces and with said opening progressively increasing from a minimum proximate said one end to a maximum at said opposite end, an input portion of said coaxial line at said one end of said termination portion, and a member of microwave absorbing material disposed in peripheral proximity to said other of said surfaces of said termination portion.

9. Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer tubular shield, a termination portion of said coaxial line coiled about an axis with axially adjacent coils defining radially inner and outer surfaces, one of said surfaces of said termination portion tapered radially between one end and an opposite end, the other of said surfaces of said termination portion extending axially substantially radially uniformly with said outer shield of said coaxial line being partially removed to define an opening facing' radially lfrom said other'of'said-surfaces and with y Y progressively increasingVfr-om aY minimum Vproximate said said opening progressivelyincreasing from Ya minimum proximate saidone-end to.an1aximum atV saidV opposite endsaidopening. extendingfor a length at'lleast equal to twice the wave'leng'th ofthe lowest microwave .fre'qilency' to be absorbed, an input portion of Ysaid coaxial hne at said'one end of said termination portion, and a member.v

of microwave absorbing material disposed in peripheral proximity ,to'saidV otherof saidpsurfaces-of said' terminationl portion. A y Y 10. Apparatus for absorbing incident power of electromagnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded l opposite endto a maximurnat said .0ne-end, :an input ypor- `tion of saidl Y.coaxial line at said opposite end of said termination portion, and a member of microwave absorbing material disposed in peripheral proximity to said outer4 surface of said termination portion.

13. Apparatus for absorbing incident power of electrof magnetic waves withingthe microwave spectrum comprising a generallytubular housing, acoaxial line disposed, in-said housing and having a center conductor surrounded. by an outer tubular-shield, a'termination portion of said coaxial line coiled 'about any axis with axially adjacent by an outer tubular shield, a termination portion of ksaid coaxialY line coiled about an axis with axially adjacent coils defining radially inner and outer surfaces, one of said Surfaces of said terminationfportion: tapered radially between one end and an opposite end,vthe other of saidv surfaces of said termination portion extending axiallysubi stantiallykradially uniformly, said coaxial line ofy said,

terminationportion decreasing in cross-sectional area gradually from one endl'oi"A said termination portion toY approximately one-half its original cross sectional area' atan opposite end with said 'outer shield of-y said coaxial line` being partially removed toy define anropening facing radiallyl kfrom saidother surface and with saidopeningV progressively increasing from a minimum proximate said one endk to a maximumat said opposite end, an input por-.7.

c oils defining radially'inner and outer-surfaces, said outer surfacelof said termination portion tapered radially inwardly from one end toward an opposite end, said inner surfaceof ,said termination portion extending axially sub-l stantially radially uniformly with said'outer shield of said coaxial line being partially lremoved to define an opening facing radially inwardly and with said opening progres- 's'ivelyincreasing from a minimum proximate. said one en d to a maximum at said opposite end, an input portion of said coaxial line at said one end of'said termination portion, a member of microwave absorbing material disposed. in peripheral proximity to said inner surface of said termination portion,l and means operatively associated with said member, said'housingvand lsaid coaxial line for movj ing said member axially Yrelative to said termination portion.

tion. of said'coaxial lineart said one end of saidY termina.

tion-portion, and a member of microwave absorbing-mato said other of" terial Ydisposed iny peripheral proximity said surfaces of said termination portion. a

' 11, Apparatus for absorbing incident power of'electro-VV magnetic waves within the microwave spectrum comprising a coaxial line having a center conductor surrounded by an outer'tubular shield, a termination portion of said@ .j coaxial line coiled 'about an axis with axially adjacent coils dening radially inner and 'outer surfaces, said outerk Y surface offsaid termination portion tapered radially. in-

wardly from one end toward anopposite end, said 'innerY surface of said termination portion rextending axially substantially radially uniformly with said outer-:shield ofvsaid coaxial line-being partially removed' to` define an openingvr facing radially inwardly and with said opening progressively increasing from a minimum'proximate said one endYV r to a maximum at ,saidoppositeend', an input. portion of c said coaxial line at'said oneend of said terminationk por tion, anda member of microwave absorbing material dis-v to saidinner surface of said posedin peripheral proximity termination portion. v

12. Apparatus for absorbing incident power of electromagne'tic'waves within the microwavespectrum comprisV ing a coaxial line having a center conductor surroundedby an. outer tubular shield, a termination portion of said" coaxial line coiled" about Van axis with axially adajcent Y coils defining radially inner and outer surfaces, said inner surface of said termination Iportion taperedr radially inwardly from one; end toward an opposite end, said'outer surface of said termination portion extending axially sub'- stantially radially uniformly with said outer shield'ofA said coaxial rline being partially removed' to V-defineV an opening facing radiallyV outwardly andwith said opening openingfacing radially outwardly and with said opening progressively increasing from a'minimum proximate said opposite-end to a maximumy at said one end, an input' 'n portionV of Vsaid coaxial line at said opposite end of said material disposedin peripheral proximity to said outerl termination portion, amember Vofmicrowave absorbing surface of said termination portion, and means operative- .ly associated with said member, saidfhousing, and said d coaxial linegfor moving said member axially relative to said termination portion. Y

. References. Cited by the Examiner UNITED STATES PATENTS 2,968,775 l6/61 Rose ---p v S33-22 3,079,552; 2/63 Walker 338-84 X 3,092,793 6/631' vi'fx'ugustine'etY al. 333f-3'1 f d OTHER-REFERENCES Montgomery: Techniquel of Microwave Measurements,

d Ypage V734,'McGraw-HilL-New York. (Copyright 1947;)

HERMAN-.KARL sAA-LBACH, Primary Examiner.

UNITED STATES .PATENT DFFICE CERTIFICATE 0F CURRECTIGN Patent No 3,176, 248 March 30, 1965 VincentJ, McHenry It is hereby certifiedthat error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line. 48, after "providing" insert for column 4, line ll, for "frequence" read frequency column 5, lines 26 and 4l, after "disposed", each occurrence, insert proximate Signed and sealed this 24th day of August 1965.,

(SEAL) Attest:

ERNEST w. swIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. APPARATUS FOR ABSORBING INCIDENT POWER OF ELECTROMAGNETIC WAVES WITHIN THE MICROWAVE SPECTRUM COMPRISING A COAXIAL LINE HAVING A CENTER CONDUCTOR SOURROUNDED BY AN OUTER TUBULAR SHIELD, A TERMINATION PORTION OF SAID COAXIAL LINE, SAID OUTER SHIELD OF SAID COAXIAL LINE OF SAID TERMINATION PORTION BEING PARTIALLY REMOVED TO DEFINE AN OPENING PROGRESSIVELY INCREASING FROM A MINIMUM PROXMATE AND INPUT END OF SAID TERMINATION PORTION TO A MAXIMUM AT AN OPPOSITE END, AND A MEMBER OF MICROWAVE ABSORBING MATERIAL DISPOSED PROXIMATE TO AND IN CONFRONTATION WITH SAID OPENING. 